Harden codebase and add documentation stubs

[zilchms]

Might be backwards incompatible due to codebase hardening. Hardening occurs in:

• repo::apt
• repo::rhel
• service
  All of those are either changing String to String[1] (disallowing empty Strings) or add a String datatype to variables already having a default String value

[wyardley]

Hi @zilchms
Can you clarify what is being hardened and what the likely breaking changes might be? Are they related to features not available in certain Puppet versions, or something else?

[wyardley]

Also, I'm thinking this should probably be flagged as an enhancement as well as a docs change?

[zilchms]

Sure let me edit the description real quick :)

[wyardley]

LGTM 🙏

Not sure if this will cause big conflicts with #926 -- @kajinamit - you may have to make some adjustments when you rebase.

[wyardley]

I'm going to leave off the "backwards incompatible" label, since I don't think this should interfere with normal / expected use of the module; let me know if that's untrue, though.